Abstract: Mode localization and transition in spatial structures are studied in this paper.Through the modal analysis of a plane substructure of spatial structures,the existence of mode localization and transition is verified.The mechanism of two phenomena is rerealed by the perturbation theory.The conclusion is that the internal cause of two phenomena is intensive frequencies of spatial structures and the external cause is the slight change of structural physics parameters.At last,the important impact of two phenomena is expounded in the areas of earthquake resistance calculation and damage identification of spatial structures,and these phenomena are suggested to be considered adequately.It is showed that the earthquake resistance calculation without considering mode localization may be unsafe.And it may be an effectual approach to solve the problem of damage identification of spatial structure by mastering and using mode localization and transition phenomena.